The subject matter disclosed herein relates to a circuit breaker having a series arc fault detector and method of operation. In particular, the subject matter disclosed herein relates to a series arc fault detector and method of operation where the sensitivity level and enable threshold for detection of a series arc fault may be changed in response to changed operational conditions.
A parallel arc is a type of electrical fault that can occur as a short circuit or a ground fault. A short circuit arc decreases the dielectric strength of insulation separating the electrical conductors, allowing a high-impedance, low-current arc fault to develop. Over time the arc carbonizes the conductor's insulation causing further decreases in the dielectric strength of insulation that separates the electrical conductors. The result is increased current and greatly increased thermal energy. During a short circuit, the system impedance and the impedance of the arc fault itself limit the current flow through a parallel arc fault.
In contrast, a series arc fault is a type of fault that can occur when an electrical conductor in series with a load is unintentionally broken. For example, a frayed conductor in a cord that has pulled apart, a loose connection to a receptacle or a loose connection in a splice. A series arc is load limited, meaning that the arc current cannot be greater than the load the electrical conductor serves. The current that flows through an arc in series has a lower root mean square (RMS) value than current without the arc due to extinction and re-ignition.
Due to the lower levels of RMS current that occur in series arc fault, detecting the presence of the series arc is difficult. In addition, the series arc must also be differentiated from electrical arcs that are normally generated by common household appliances, such as vacuum cleaners, hand drills and light switches for example. Unfortunately, due to the wide variety and conditions that may occur in a residential electrical circuit, series arc fault detectors have a high level of false positive detections that tend to result in undesirable nuisance tripping of the circuit breaker.
Series arc fault detection methodologies typically involve the interrogation of certain frequencies in the electrical current. Typically, a microprocessor monitors these frequencies and increments a counter when the suspect frequencies are detected. The frequencies are compared against a threshold, such as the number of frequency peaks accumulated during a period of time for example. Once the threshold is exceeded, the circuit breaker opens and provides an indication to the operator that a series arc fault has been detected. Other detection methods use wavelet onset time domain methodologies. These wavelet methods generally determine the duration of a half cycle of the current and determine if time durations measured between successive pairs of the wavelets during a predetermined time period match multiples of the duration of the half cycle of the current. In either of these methods, the sensitivity of the arc fault detector may be changed by modification of the detection algorithm parameters and the thresholds.
Series arc fault detectors are typically tested under procedures promulgated by a standards agency or body, such as Underwriters Laboratory (UL) procedure 1699 for example. Under these procedures, the detector needs to detect a series arc when the current passing through the circuit breaker is 5 amps or above. However, manufactures typically lower their detection threshold to 3.5 amps in order to assure compliance with the testing procedures.
Commercially available arc fault detectors are usually set to a high level of sensitivity to avoid missing a potential series arc fault. It should be appreciated that the number of appliances and conditions that the circuit breakers are exposed to in the field is large, and since circuit breakers usually operate 24 hours a day, seven days a week, the opportunity for a condition that looks like a series arc fault to the detector is high. When a false positive detection is generated, the circuit breaker is unnecessarily tripped causing a loss of power to the protected circuit.
While existing circuit breakers are suitable for their intended purposes, there still remains a need for improvements particularly reliability of detecting series arcs and reduction of nuisance tripping of the circuit breaker.